Radio frequency filter



March 21, 1961 s. s. LOCUS 2,976,498

RADIO FREQUENCY FILTER Filed Aug. 3, 1959 2 Sheets-Sheet 1 FIG. 1.

FREQ- INVENTOR.

BY Km AGENT SILVAN S. LOCUS 1 March 21, 1961 Filed Aug. 3, 1959 2 Sheets-Sheet 2 6 65 RECEIVERI FILTER eo AN ENNA RECEIVER 2 39 |9 INVENTOR.

SILVAN S. LOCUS AGENT RADIO FREQUENCY nurse Silvan S. Locus, Van Nuys, (Ialif, assignor to Electronic Specialty (30., Los Angeles, Calif, a corporation of California Filed Aug. 3, 1959, Ser. No. 831,159

11 Claims. (Cl. 333-99) My invention relates to an electrical filter in which the physical size of the elements determines the operating frequency and particularly as to the structural arrangement thereof.

In the megacycle and the kilo-megacycle regions of the radio frequency spectrum the use of resonant elements with distributed constants is an efficient and convenient manner of forming physically realizable structures. However, the ordinary linear disposition of such elements usually results in a large and an unwieldy structure. Accordingly, I have invented a very much different structure in which the elements are combined in a convenient and compact structure having a maximum dimension roughly one-third the maximum dimension of the known arrangement. I also attain important advantages of adjustability, freedom in positioning connectors for external connections, high Q for elements when this is of advantage and improved performance under conditions of vibration and extremes of temperature.

Briefly, I am able to alter a characteristically long, wide and thin filter of this class into What can be a cube of reduced volume by separating the physical structure of the half wavelength sections from that of the one-quarter and three-quarter wavelength strip lines. The latter lines connect to the external apparatus, whereas the half wavelength sections are capacitatively bridged between the strip lines. The Q of the strip lines is of lesser consequence than that of the half wavelength sections and by decreasing the cross-section of the strip lines I am able to obtain superior electrical performance within a reduced volume.

Because the one-quarter and three-quarter wavelength strip lines are of small cross-section I am able to easily fold or otherwise change the configuration thereof to reduce the volume and to improve the physical shape of the filter. I am also able to provide connective means to external circuits at a variety of positions around the rectangular parallelepiped as may be desired in practical embodiments.

Illustrative embodiments include a two section filter having two notches in the frequency characteristic feeding a common element,.such as an antenna from two transmitters, receivers or transceivers. Between one such entity and the antenna the attenuation characteristic is low at the two notch frequencies so that that entity is, in effect, connected to the antenna at these frequencies only. Beween the other entity and the antenna the attenuation characeristic is low at all frequencies in the general region of the spectrum of interest except at the two notch frequencies, where it is relatively very high. Independent and simultaneous operation (diplexing) of the two entities from the single antenna is thus afforded.

It will be understood that the two above-mentioned entities may be termed a first active radio frequency devicef and a second active radio frequency device, respectively, and that the single antenna may be termed a common radio frequency element. The terms operative radio frequency device and radio frequency instru- ICE mentality common to both said devices will also be recognized as synonyms.

An object of my invention is to provide a greatly improved physical form for a radio frequency electrical filter.

Another object is to improve the electrical performance of such a filter while decreasing its physical volume.

Another object is to provide such a filter in which the electrical balance and the exact frequencies of operation may be adjusted subsequent to its manufacture.

Another object is to provide an embodiment of such a filter as allows flexibility in the placement of connectors therefrom to external circuits.

Another object is to provide such a filter having improved electrical performance under conditions of vibration and extreme temperatures.

Other objects will become apparent upon reading the following detailed specification and, upon examining the accompanying drawings, in which are set forth by way of illustration and example certain embodiments of my invention.

Fig. 1 shows a side sectional elevation of my filter,

Fig. 2 shows a sectional plan view thereof taken through essentially the central plane thereof,

Fig. 3 shows a sectional end elevation of the same,' j

Fig. 4 shows an alternate embodiment with an air insulated central line in the same sort of View as Fig. 2,

Fig. 5 shows the attenuation characteristics of my filter as a function of frequency, and

Fig. 6 shows one manner in which my filter may be used.

In Figs. 1 and 2 numeral 1 indicates connection means, such as a standard coaxial type connector suitable for radio frequencies. To it is connected a common element, such as an ultra-high frequency antenna, and for purposes of illustration it will be assumed that radio frequency energy is incoming, as in a receiving operation.

Connection means 1 has an inner conductor 2. The latter connects to one end of a strip line center strip 3, which line will be seen to have a center section 4 that is one-quarter wavelength long at the attenuation transfer frequency of the filter. The other end 5 of this line connects to another connection means 6, which leads to a radio frequency operative device (a receiver in this example) to be supplied with incoming energy at all frequencies save two fairly sharply defined frequency increments 7 and 8 as represented by attenuation peaks in Fig. 5. v

The two ground planes required as the outer conductors of the strip line are 9 and 10. These extend over the whole cross-sectional area; i.e., the horizontal plane of Fig. 2. In the embodiment of Figs. 1 and 2, two slabs of Teflon 11 and 12 are sandwiched in between the planes 9 and 10 and occupy all of the space between these and the centrally disposed strip line 3, 4, 5. The dimensions of this portion of my device and of the connecting means are chosen to give a uniform impedance, as 50 ohms.

The fiat and extensive configurations of the prior art have employed relatively wide strip line throughout in order to provide a high Q to certain sections and so have required an area many times that shown in Fig. 2.

Contrariwise, I employ capacitative probe 14 to convey radio frequency energy from the strip line configuration to volumes 15 above and 16 below that configuration. These, volumes, in coaction with half wavelength sections 17 and 18 are coaxial line resonators. The walls defining each volume act as the outer conductor and the conductive sections as the inner conductor in each case. In Figs. 1 and 3 particularly, the existence of four such volumes is to be noted. These are: the upper forward volume bounded by plane 9 on the bottom, partition 21 at the rear, cover 19 at the top and plate 23 at the front;

the lower forward volume bounded by plane 10 at the top, bottom 20 at the bottom, partition 22 at the rear and plate 24 at the front; the upper rear volume bounded by cover 19 at the top, plane 9 at the bottom, partition 21 at the front and plate '25 at the rear; and the lower rear volume bounded byplane 11 at the'top, bottom 20 at the bottom, partition 22 at the front'and plate 26 at the rear. These plates and partitions are fastened together with numerous screws as known in the art, which screws have not been shown for sake of clarity. It is possible to form several'of these members together, as 23, 19, 25, and as another group 9 and 21, etc. '111 combining'the members it is only necessary that it be possible to insert the internal elements in each volume.

Half wavelength sections 17 and 27 form a two section filter to provide the attenuation peak at one'frequency, as 7 in Fig. 5. Similarly, half wavelength sec tions .18 and 23 form a second two section filter acting to form the attenuation peak at the higher frequency 8. One pair of half wavelength sections is tuned by the adjustment of the tuning elements, as 41, 43, slightly differently than the other pair, and so two attenuation peak frequencies result.

Capacitative probe 14 is electrically connected to strip line 3 and passes through a hole in ground planes 9 and 10. The presence of such a probe causes a shunt capacitance to exist between the strip line and its ground planes 9 and 10. Such a capacitance, if not corrected, constitutes a discontinuity on the strip line 3 and would cause reflection of the radio frequency waves on this line. The effect of this capacitance is corrected by choosing an appropriate diameter for the openings in the ground planes through which probe 14 passes, removing the Teflon directly within the holes and reducing the width of strip line 3 in the vicinity of each hole. Epoxy-bonded glass board wafers 29 and 30 are used to support the probe 14 and these are made thin so as to add only slightly to the shunt capacitance of the probe. These several compensatory measures result in an essentially refiectiouless (smooth) strip line.

Each half wavelength section, as 17, is supported at three points by pillars of insulation, such as 31, 32, 33. These may be fabricated of cross-linked polystyrene or an equivalent dielectric and cemented with an epoxy or equivalent effective cement to the metal surfaces involved, 9 and 19. The half wavelength sections are made in the form of a shallow U so as to be structurally stiff. The whole construction is similarly ruggedly fabricated so as to give stable electrical characteristics under conditions of vibration or extremes of temperature. Probe 14 has been shown somewhat away from the su1faces of sections 17 and 18. This was for sake of clarity, and in actual practice the probe is closer to these sections to increase the capacitance thereto. An equivalent capacitative probe 34 conveys electrical energy also from the quarter wavelength portion of strip line 4, S to half wavelength sections 27 and 28.

An oppositely disposed important element of the filter is comprised of three-quarter wavelength strip line 35 and associated capacitance probes 36, 37. End 38 of this line connects to external connection means 39. To means 39 there is connected the second. radio frequency operative device. This one receives energyfrom common element 1 only in the vicinity of peaks 7 and 8; i.e., according to curve 56 of Fig. which shows minima of attenuation at these frequencies.

Element 40 is a. small slab of resistive material-and forms a non-reflective termination at the upper end of three-quarter wavelength strip line 35. This resistor has the same resistance as the characteristic impedance of the strip line; 50 ohms in a typical embodiment.

A total of eight capacitor tuning elements are'preferably also included as a part of the half wavelength sections in my filter. In Fig. 1 four of these can be seen; 41, 42, 43, 44;'and in Fig. 3, two more; 45, 46. These provide the filter.

a desirable flexibility of adjustment to my filter. In the first place, the capacitance to the half wavelength sections can be altered by screwing upward or downward the threaded stem, as 47 in Fig. l, to alter the resonant frequency of this half wavelength section 17. It is thus possible to remove the effects of inaccurate workmanship in the fabrication of the filter. This both reduces the cost of manufacturing the same and allows a degree of perfection of adjustment and performance not obtainable by an ordinary configuration of strip lines alone. Secondly,

the location of the attenuation peaks may be altered in frequency by altering all four capacitors on one side of In practice, locknuts such as 48 are provided for setting these adjustments and these may be soldered tight when the proper adjustment has been obtained in manufacturing for the trade.

With my enlarged configuration of half wavelength sections a relatively-high Q is obtained. This is of the order of 60 when loaded and several thousand per se, unloaded.

The strip line portion of my filter operates in the TEM (transverse electromagnetic mode), as do the other half wavelength sections also. It is desirable that waveguide modes not be excited in the cavity formed between planes 9 and 10. It has been found in practice that this does not occur. However, for additional precaution it is merely necessary to add a few pins or screws as conductors transverse of the major dimension of this central volume, as at 50. These pass through the volume and are electrically connected to plane 9 and/or 10.

The desired filtering effect is obtained when either pair of the half wavelength sections 17, 27; 18, 28 are resonant. This switches the incoming power from one terminal to the other. Assume that the incoming radio frequency power is from a receiving antenna, as has been previously suggested, and that connection thereto is made at connector 1. At radio frequencies other than the resonant frequencies of either pair of half wavelength sections the incoming energy merely passes around quarter wavelength strip 3, 4, 5 and appears at connector 6.

At resonance of a pair of the half wavelength sections of a given resonant frequency the equivalent of a conductive electrical connection occurs between capacitative probes 14 and 36 and between probes 34 and 37. The only exception to the literal interpretation of this statement is that the phase of the electrical energy is shifted through (rather than no phase shift as if conductors were actually placed between the points mentioned). This situation has the following effect on the filter.

A pair of the half wavelength sections, say 17 and 27, being resonant; power is transferred from connector 1 through probe 14, section 17, probe 36 and out connector 39 to one active radio frequency device connected there to; as a receiver.

Conversely, the circuit as a whole is such that voltage nulls appear at probes 34 and 37; thus power is not transmitted to either connector 6 or to resistor 40. Another active radio frequency device is connected to connector 6, as another receiver. Resistor 40 is not included as a fundamental part of the filtering circuit, but merely to preserve the characteristic impedance thereof in spite of manufacturing departures from a fully symmetrical and relatively ideal structure.

The null appears because of the difference in length in the two paths between probes 14 and 37. One such path is from probe 14 to probe 36, a half Wavelength via resonant section 17, plus the three-fourths wavelength from probe 36 to probe 37. This is a total of five-fourths wavelength. The second such path is a quarter wavelength from probe 14 to probe 34, plus a half wavelength via resonant section 27. This is a total of'three-fourths wavelength. The difference between these two paths is a half wavelength and thus voltage cancellation occurs. This prevents dissipation in resistor 40.

A similar condition obtains between probes 14 and 34.

The short path is a quarter wavelength; through strip 4. The long path is from probes 14 to 36, a half wavelength; plus the strip from 36 to 37, three-fourths wavelength; plus from probes 37 to 34, a half wavelength. The total path is seven quarter wavelengths. The difference be tween a quarter wavelength and seven quarter wavelengths is one and one-half wavelengths. Since a full Wavelength represents a return to original conditions it is of no moment in considering matters of phase opposition. A half wavelength thus remains as the significant difierence in path length. Again, this is phase opposition and therefore voltage cancellation. Power is thus prevented from reaching the active radio frequency device connected to terminal 6. I

The situation outlined above causes the attenuation characteristic to be at one of the peaks, say 7, as shown in Fig. 5. When half wavelength sections 18 and 28 are resonant, at a somewhat different selected frequency, the attenuation characteristic is that of the other peak, 8. At frequencies away from these resonances the attenuation characteristic between connectors 6 and 39 with respect to connector 1 is reversed, as shown by curve 56 of Fig. 5.

My filter may be used in various ways, one of which is shown in Fig. 6. An exterior view of filter 19 is shown in the same plan as in Fig. 2 or 4. It has been previously mentioned that a common radio frequency element is connected coaxially to connector 1. This is illustrated as an antenna system comprised of antenna 60, antenna ground plane 61 and coaxial cable 62. The antenna is connected to the internal conductor 63 of the coaxial cable and the ground plane to the external conductor of the coaxial cable. Known matching systems between the coaxial cable and the antenna may be employed but have not been shown.

In a like manner, a first active radio frequency device is represented by Receiver 1, 64; which is connected to coaxial cable connector 6 by coaxial cable 65. Similarly, a second active radio frequency device is represented by Receiver 2, 66; which is connected to connector 39 by cable 67. As has been previously brought out, Receiver =1 is connected to the antenna at all frequencies save the two peak frequencies 7 and 8 in Fig. 5. Conversely, Receiver 2 is supplied energy from the antenna at only the peak frequencies '7 and 8.

As was also initially mentioned herein, the two receivers shown may instead be two transmitters or two transceivers, and the antenna may be of a different type or may be another kind of common load, or a source or an absorber of electrical energy.

An alternate embodiment of my filter is shown in Fig. 4. Here the central one-quarter and three-quarter lines are supported in air rather than being between a solid insulator, as in Fig. 1. The remainder of the structure is substantially as shown in Fig. 1. Corresponding parts in the second embodiment have been given prime sufiixes of the antecedent thereof in the first embodiment. Accordingly, the one-quarter wavelength line connects to connection means 1, has portions 3', 4', and 5', and leaves through connection means 6, as before. However, instead of being of strip form the line is of circular form (of the order of an eighth inch in diameter) and is slightly longer than the previous strip. This is because the wavelength on such a line in air is greater than the same in a solid dielectric having a dielectric constant greater than one. Where capacitative probes 14 and 34 are attached the diameter of line 4 is reduced so that the impedance thereof shall remain substantially constant.

The line 3', 4', 5' is supported from plane by four pieces of solid insulation 52. The line is also reduced in diameter where these pieces of insulation are attached in order to provide constant impedance. The pieces of insulation may be formed of cross-linked polystyrene (Rexolite) or of equivalent radio frequency insulation.

Three-quarter wavelength line 35' is similarly constructed and supported. Eight pieces of insulation'53 are preferably employed for support and the longer path of the air dielectric three-quarter wavelength line is noted by the expanded shape with respect to strip 35 previously.

The central metal separators 54 and 55 add mechanical strength to the central volume in the absence of the Teflon slabs and also act to suppress wave guide modes.

As before the structural screws have not been shown for sake of clarity.

Certain other modifications of my invention are possible.

Should it be desired that one or both connecting means 1 and 6, for instance, be positioned on opposite sides of the parallelepiped it is only necessary to so locate them The portions 3 and 5 of the one-quarter wavelength line then pass to the sides of the parallelepiped rather than being folded toward one end as shown in Fig. 2.

While a common antenna has been mentioned in the typical embodiments described it will be understood that any common load element into which diplexing is desired may be connected at 1 or 1'.

It will also be understood that the cross-sectional shape of the half wavelength sections need not be relatively flat as shown, but maybe relatively oval or cylindrical. Also, the internal and external cross-sectional shape of volumes 15, 16, etc. need not be rectangular. These may be oval or circular, thereby giving an oval cross-section to the external surface of the filter.

The tuning capacitor elements 41, etc. maybe omitted, although I do not prefer to do this. They may take the form of angle tabs, which may be adjusted by bending.

Normally, aluminum and copper or brass, preferably silver plated for superior radio frequency conductivity, are employed for the several conductive parts of the structure, although a wide choice of materials may be employed. For dielectric slabs 11 and 12 polystyrene, ceramic or quartz may be employed instead of Teflon.

By eliminating one pair of half Wavelength sections, as 18 and 28, only one peak and notch in the overall characteristic of Fig. 5 can be obtained. This would solve another specific type of application problem and is fully Within my teaching of structure.

Still other modifications may be made in the arrangement, size, proportions and shape of the illustrative embodiments shown without departing from the scope of my invention.

Having thus fully described my invention and the manner in which it is to be practiced, I claim:

1. A radio frequency filter comprising a conductive housing having more than two separate volumes, at onequarter and a three-quarter wavelength transmission line within one of said plural volumes, plural half wavelength transmission line sections within plural volumes adjacent to said one volume, one half wavelength section to each said volume, means to couple one end of each said plural half wavelength section to said one-quarter wavelength line and the other end to said three-quarter wavelength line through a uniform impedance structure, a first active radio frequency device connected to said three-quarter wavelength line, a second active radio frequency device connected to said one-quarter wavelength line, and a common radio frequency element also connected to said one-quarter wavelength line, said filter having the electrical characteristic of high attenuation between said first active device and said common element at all save certain frequencies adjacent to the frequency at which said half Wavelength sections are resonant, and the characteristic of low attenuation between said second active device and said common element save at said cer tain frequencies.

2. A radio frequency filter comprising a conductive housing having more than two separate volumes, one oneqnarter wavelength and one folded three-quarter wavelength strip line disposed oppositely within one of said plural volumes, plural half wavelength transmission line sections within plural volumes adjacent to said one volume, only one'half wavelength section within one said quarter wavelength line through a uniform impedance structure, capacitative means to tune said half Wavelength sections, means disposed to allow radio frequency excitation of said lines and said sections by only the transverse electromagnetic mode, a first active radio frequency device connected to said three-quarter wavelength line, a resistive termination also connected to said three-quarter wavelength line, a second active radio frequency device connected to said one-quarter wavelength line and a passive radio frequency element also connected to said one-quarter wavelength line, said filter having the electrical characteristic of high radio frequency attenuation between the first said active device and said passive element at all save certain frequencies adjacent to the he quency at which said half wavelength sections are resonant, and the characteristic of W radio frequency attenuation between said secondactive device and said passive element save at said certain frequencies, the radio frequency attenuation first mentioned being low at said certainfrequencies and the attenuation secondly mentioned being high at said certain frequencies.

3. The filter of claim 2 in which a said means to capacitatively couple comprises a conductor connected to a quarter wavelength line at a reduced dimension of said line, said conductor passing through a hole in said conductive housing leading from one said separate volume to another, and a thin insulating diaphragm supporting said conductor from said housing at said hole.

4. A radio frequency filter comprising an electrically conductive housing, conductive partitions to form a shallow volume having on one side two equal volumes, a folded one-quarter wavelength transmission linedisposed at one end of said shallow volume, a three-quarter wavelength transmission line disposed at the opposite end of said shallow volume, a half wavelength section within each of said equal volumes, means to capacitatively couple said one-quarter wavelength line to one end of each of two said half wavelength sections, means to oapacitatively couple said three-quarter wavelength line to the opposite end of each two said half wavelength sections, each of said one-quarter, three-quarter and half wavelength entities coactive with said housing to convey radio frequency energy by the transverse electromagnetic mode, said means to capacitatively couple formed with respect to said housing to maintain a constant impedance upon said one-quarter and said three-quarter wavelength lines; a first radio frequency device connected to said three-quarter wavelength line, a second radio frequency device and a radio frequency element common 'to both said devices at mutually exclusive frequencies connected to said one-quarter wavelength line; the attenuation between said first radio frequency device and said common element being high for all save one frequency band ad jacent to the frequency at which said half wavelength sections are resonant, and the attenuation between said second radio frequency device and said common element being low for all save said one frequency band.

5. A radio frequency filter comprising a conductive housing having a major dimension, conductive partitions parallel to said major dimension to form a central volume surrounded by four equal volumes, a one-quarter wavelength transmission line synnnetrically disposed within said central volume, a three-quarter wavelength transmission line similarly and oppositely disposed within said central volume, each of said one-quarter and said threequarter wavelength transmission lines having a non-linear three-quarter wavelength line to the opposite end of each said four half wavelength sections, each of said one-quarter, three-quarter and half wavelength transmission lines coactive with said housing to convey electrical energy of radio frequency, said means to couple formed with respect to said housing to maintain a constant impedance upon said transmission lines; a first radio frequency device connected to said three-quarter wavelength line, a second radio frequency device and a radio frequency element common to both said devices at mutually exclusive frequencies connected to said onequarter wavelength line; the attenuation between said first radio frequency device and said common element being high for all except certain frequencies adjacent to the frequency for which said half wavelength sections are resonant, and the attenuation between said second radio frequency device and said common element being low for all frequencies except said certain frequencies.

6. The filter of claim 5 in which the Q of said half wavelength sections is greater than the Q of said onequarter and of said three-quarter wavelength lines.

7. A radio frequencyfilter comprising an electrically conductive housing, plural conductive partitions to form a thin central volume surrounded on opposite sides by two equal volumes, only one one-quarter wavelength transmission line symmetrically disposed at one end of said central volume, only one three-quarter Wavelength transmission line similarly disposed at the opposite end of said central volume, a half wavelength section of transmission line within each of said equal volumes, each said half wavelength section having a larger cross-section than either of said one-quarter or said three-quarter wavelength lines, means to capacitatively couple said one-quarter wavelength line to one end of each of said half wavelength sections, means to capacitatively couple said three-quarter wavelength line to the opposite end of each of said half wavelength sections, a capacitor element connected between each end of each of said half wavelength sections and said housing; each of said one-quarter, three-quarter and half wavelength entities coactive with said housing to convey radio frequency energy by the transverse electromagnetic mode, said means to capacitatively couple formed with respect to said housing to maintain a constant impedance upon said one-quarter and said three-quarter wavelength lines, resistive means connected to one end of said three-quarter wavelength line, a first radio frequency device connected to the other end thereof, a second radio frequency device connected to said one-quarter wavelength line opposite said resistive means, and a radio frequency element common to both said devices at mutually exclusive frequencies connected to the other end of said quarter Wavelength line; the radio frequency attenuation between said first radio frequency device and said common element through said filter being high for all frequencies except two frequency bands adjacent to that frequency at which said half wavelength sections are resonant, and the attenuation between said second radio frequency device and'said common element being low for all frequencies except said two frequency bands.

8. A radio frequency filter comprising an electrically conductive housing, two conductive planes bisecting said housing to form three volumes, a one-quarter wavelength transmission line disposed in the central of said three volumes, a three-quarter Wavelength transmission line similarly oppositely disposed, two partitions disposed to bisect the volumes within said housing above and below said conductive planes into four substantially equal volumes, a half wavelength section of transmission line within each substantially equal volume, a capacitatively effective probe conductively attached to said quarter wavelength line and coupled to two said half wavelength sections at one end of each, and a second capacitative'probe attached to said quarter wavelength line and coupled to the two further said half wavelength 9 sections at the ends thereof corresponding in position within said housing to the ends of the two previously mentioned half wavelength sections, a third capacitative probe attached to one end of said three-quarter wavelength line and coupled to said two half wavelength sections at the ends thereof opposite said capacitative probe attached to said quarter wavelength line, and a fourth capacitative probe attached to the other end of said three-quarter wavelength line and coupled to said further half wavelength sections at the ends thereof corresponding to those associated with said third capacitative probe; a first radio frequency device connected to the other end thereof, a second radio frequency device connected to said quarter wavelength line opposite said load, and a radio frequency instrumentality common to both said devices connected to the other end of said quarter wavelength line; the attenuation between said first radio frequency device and said common instrumentality through said filter being high for all save two narrow frequency bands adjacent to the frequency at which said half wavelength sections are resonant, and the attenuation between said second radio frequency device and said common instrumentality being low for all frequencies save sm'd two narrow frequency bands.

9. A radio frequency filter structure comprising a rectangular parallelepiped electrically conductive housing, two conductive planes symmetrically bisecting said housing to form three volumes each of uniform thickness, one'one-quarter wavelength transmission line insulatingly centrally disposed in the central of said three volumes near one end thereof, one three-quarter Wavelength transmission line similarly disposed near the other end thereof, means to inhibit waveguide modes of energy propagation in said central volume, two partitions perpendicular to said two conductive planes disposed to bisect the vol umes within said housing above and below said conductive planes into four equal volumes, a half wavelength transmission line section insulatingly mounted within each said equal volume, a capacitative probe conductively attached to one end of said quarter wavelength line and coupled to two said half wavelength sections at one end of each, and a second capacitative probe attached to the other end of said quarter wavelength line and capacitatively coupled to the two further said half wavelength sections at the ends thereof corresponding in position within said housing to the ends of the two previously mentioned half wavelength sections, a third capacitative probe attached to one end of said three-quarter wavelength line and capacitatively coupled to said two half wavelength sections at the ends thereof opposite said capacitative probe attached to said quarter wavelength line, and a fourth capacitative probe attached to the other end of said three-quarter wavelength line and capacitatively coupled to said further half wavelength sections at the ends thereof corresponding to those associated with said third capacitative probe; eight capacitor elements, each of said elements connected to said housing adjacent to and in capacitative relation to one end of each said four half wavelength sections for adjustment of the frequency response of said filter; a dissipative load having an impedance equal to that of said three-quarter wavelength line connected to one end thereof, a first radio frequency operative device connected to the other end thereof, a second radio frequency operative device connected to said quarter wavelength line opposite said load, and a radio frequency instrumentality common to both said devices at mutually exclusive frequencies connected to the other end of said quarter wavelength line, the attenuation between said first radio frequency device and said common instrumentality being high for all frequencies save two frequency increments adjacent to the frequency at which said half wavelength sections are resonant, and the attenuation between said second radio frequency device and said common instrumentality being low for all save said two frequency increments.

'10. A radio frequency filter comprising an electrically conductive housing, two conductive planes bisecting said housing, insulation disposed between said planes, a onequarter wavelength strip folded and centrally disposed within said insulation near one end thereof, one threequarter wavelength strip folded and disposed within said insulation near the other end thereof, partitions disposed to bisect the volumes within said housing above and below said'conductive planes into four equal volumes total; a half wavelength section within each said equal volume, a capacitative probe conductively attached to said quarter wavelength line and coupled to two said half wavelength sections at one end of each, and a second capacitative probe attached to said quarter wavelength line and coupled to the two further said half wavelength sections at the ends thereof corresponding in position within said housing to the ends of the two previously mentioned half wavelength sections, a third capacitative probe attached to one end of said three-quarter wavelength line and coupled to two said half wavelength sections at the end thereof opposite said capacitative probe attached to said quarter wavelength line, and a fourth capacitative probe attached to the other end of said three-quarter wavelength line and coupled to said further half wavelength sections at the ends thereof corresponding to those associated with said third capacitative probe; first connective means connected to the other end thereof, second connective means oppositely disposed to said first connective means and connected to said quarter wavelength line, and third connective means connected to the other end of said quarter wavelength line; said filter adapted to couple two separate entities operative at radio frequencies approximating the resonant frequency of said half wavelength sections to a common element, said second connective means adapted for connection to said common element, the attenuation therebetween and one said separate entity connected to said first connective means being high at all save two adjacent frequency intervals, and the attenuation between the other said separate entity connected to said third connective means being low at all save said two adjacent frequency intervals.

11. A radio frequency directional filter structure comprising a rectangular parallelepiped electrically conductive housing, two conductive planes symmetrically bisecting said housing, two slabs of insulation disposed between said planes, one one-quarter wavelength strip line centrally disposed between said slabs of insulation near one end thereof, one three-quarter wavelength strip line disposed between said slabs of insulation near the other end thereof, two partitions perpendicular to said two conducting planes disposed to bisect the volumes within said housing above and below said conductive planes into four equal volumes total; a half wavelength section of transmission line inner element insulatingly mounted within each said equal volume, a capacitative probe conductively attached to one end of said one-quarter wavelength line and capacitatively coupled to two said half wavelength sections at one end of each, and a second capacitative probe attached to the other end of said onequarter wavelength line and capacitatively coupled to the two further said half wavelength sections at the ends thereof corresponding in position within said housing to the ends of the two previously mentioned half wavelength sections, a third capacitative probe attached to one end of said three-quarter wavelength line having a capacitative coupling to two said half wavelength sections at the ends thereof opposite said capacitative probe attached to the other said end of said quarter wavelength line, and a fourth capacitative probe attached to the other end of said three-quarter wavelength line capacitatively coupled to said further half wavelength sections at the ends thereof corresponding to those associated with said third capacitative probe; eight capacitor elements, each of said elements connected to said housing adjacent to-and in capacitative relation to one end of one of said four half wavelength sections for adjustment of the frequency response of said *filter by the adjustment of the capacitance value of said capacitor elements; a dissipative load having an impedance equal to that of said threequarter wavelength line connected to one end thereof, first connective means connected to the other end of said three-quarter wavelength line, second connective means oppositely disposed to said first connective means and connected to said quarter wavelength line, and third connective means connected to the other end of said quarter Wavelength line; said directional filter adapted to couple two separate entities operative at radio frequency ranges approximating the resonant frequency of said half wavelength sections to a common element Without interaction, vsaid second connective means adapted for contween the other said separate entity connected to said 'third connective means being low at all save said two adjacent frequencies.

References Cited in the file of this patent UNITED STATES PATENTS 2,488,545 Lader Nov. 22, 1949 2,590,864 Johnson Apr. 1, 1952 2,755,447 Englemann July 17, 1956 2,808,573 Bell Oct. 1, 1957 2,859,417 Arditi Mar. 4, 19 58 

